I know I am reviving an old thread but the original question was never answered and I finally had the chance to compare the Scala head to head with the Alto the other day.
What Focal has done with the Scala is pretty impressive and something I am still having a hard time comprehending given what I thought were the similarities between the designs of the Alto and the Scala. While the Alto remains a great speaker, the Scala improves upon it in every way top to bottom and most impressively, makes the Alto seem slightly veiled in comparison (which is pretty shocking considering how veiled most other speakers sound in comparison to the Alto ime).
Were the prices the same, it would be the Scala hands down, but given the marked price differentials between the Alto on the used market and the Scala new, it's a decision that will be personal to each buyer.